I'm doing ~750kms here in Darwin with crap BP98 (not the freshest up here...), the missus can easily get over 800kms.
It's all about how you drive. I stick to driving with the cruise control on, the missus doesn't. I also accelerate a lot faster than she does.
Try driving like a grandma for a week and see how you go. Start - stop driving is the killer on fuel economy.